thermo-economic analysis of an efficient lignite-fired

IDS’2018 – 21st International Drying Symposium València, Spain, 11-14 September 2018

DOI: http://dx.doi.org/10.4995/ids2018.2018.7393

21ST INTERNATIONAL DRYING SYMPOSIUM EDITORIAL UNIVERSITAT POLITÈCNICA DE VALÈNCIA

Thermo-economic analysis of an efficient lignite-fired power system integrated with flue gas fan mill pre-drying

Han, X.a; Wang, J.a; Liu, M.a; Karellas, S.b; Yan, J.a a State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China b Laboratory of Steam Boilers and Thermal Plants, National Technical University of Athens, 9, Heroon Polytechniou Street, Zografou 15780, Greece *E-mail of the corresponding author: [email protected]

Abstract Lignite is a domestic strategic reserve of low rank coals in many countries for its abundant resource and competitive price. Combustion for power generation is still an important approach to its utilization. However, the high moisture content always results in low efficiencies of lignite-direct-fired power plants. Lignite pre-drying is thus proposed as an effective method to improve the energy efficiency. The present work focuses on the flue gas pre-dried lignite-fired power system (FPLPS), which is integrated with fan mill pulverizing system and waste heat recovery. The thermo-economic analysis model was developed to predict its energy saving potential at design conditions. The pre-drying upgrade factor was defined to express the coupling of pre-drying system with boiler system and the efficiency improvement effect. The energy saving potential of the FPLPS, when applied in a 600 MW supercritical power unit, was determined to be 1.48 %-pts. It was concluded that the improvement of boiler efficiency mainly resulted from the lowered boiler exhaust temperature after firing pre-dried low moisture content lignite and the lowered dryer exhaust gas temperature after pre-heating the boiler air supply.

Keywords: lignite; pre-drying; thermodynamic analysis; thermo-economics.

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http://dx.doi.org/10.4995/ids2018.2018.7393

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Thermo-economic analysis of the lignite-fired power sytems integrated with flue gas pre-drying


1. Introduction

Lignite is still considered as a domestic and abundant energy source for many countries. [1] Power generation is an important approach to utilization of lignite resources. However, the high moisture content always results in low efficiencies of lignite-direct-fired power plants. Pre-drying is an effective method to improve the energy efficiency because it removes the redundant moisture and increases the heating value of the lignite entering the furnace. [2,3]

Many works have been performed concerning the performance analysis of the pre-dried lignite-fired power systems (PLPSs) based on energy and exergy criteria and/or economic and environmental indicators. In China, Liu et al. [4] conducted process modeling of the steam pre-drying power system using Aspen Plus, and investigated the exergy destruction distributions of a 600 MW unit under nominal and partial loads. Moreover, biomass pre-drying [5] and solar pre-drying [6] were studied, respectively, to further demonstrate the energy saving potential of pre-drying. Zhu et al. [7] carried out an energy analysis of a lignite steam pre-drying power system with an efficient waste heat recovery system. Xu et al. [8] modeled a lignite power system integrated with boiler exhaust gas pre-drying, and analyzed the energy saving potentials with different lignite types and pre-drying degrees. Recently, different configurations of solar pre-drying were proposed by Xu et al. [9] The selection of drying heat source is gradually extended to renewable energy sources, which aims to further improve the energy efficiency of low rank coals. In Greece, Atsonios et al. [10] examined various scenarios of lignite drying and utilization depending on power load variation of the plant during certain periods of the day, in order to simultaneously increase the plant efficiency and reduce the electricity cost. Rakopoulos et al. [11] assessed different lignite-fired plant configurations for combined heat and power production for use in district heating networks, in terms of environmental, technical, and economic criteria. Avagianos et al. [12] presented a thermodynamic methodology for the prediction of lignite fired boilers at low power loads; even below the current technical minimum, when pre-dried lignite is employed. Drosatos et al. [13] conducted numerical simulation of a lignite-fired boiler using pre-dried lignite as supporting fuel for flexible operation. As summarized by Agraniotis et al. [14], state-of-the-art lignite pre-drying technologies can not only improve the energy efficiency but also operation flexibility. Energy saving of lignite pre-drying also attracts other researchers worldwide lately. For example, Fushimi et al. [15] developed drying process based on self-heat recuperation technology and evaluated the thermal efficiency penalty and costs. Kambara et al. [16] compared the thermal efficiencies for an existing 316 MWe power plant with and without steam tube dryers (STDs), and found that the improvement can be up to 4.2 %-pts. Giuffrida et al. [17] presented a detailed analyses based on mass and energy balances of lignite-fired air-blown gasification-based combined cycles with CO2 pre-combustion capture. Akkoyunlu et al. [18] carried out an economic assessment of drying prior to grinding mill process in lignite-fired thermal power plants. It was indicated that the growth in the boiler efficiency ranged between 3.9-12.6% when there

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Han, X.; Wang, J.; Liu, M.; Karellas, S.; Yan, J.


was a 10% decrease in moisture content. Moon et al [19] developed a combination of low rank coal drying with power generation, and investigated the influences of steam extraction point and amount of drying coal on the thermal efficiency. In brief, an agreement has reached that pre-drying can improve the efficiency of lignite power generation, but system optimization still requires specifications and methodologies.

The authors have reported earlier predicted energetic performance of the FPLPS at off-design conditions [20] and suggested possible performance improvement through the energy matching in the drying process [ 21]. It is necessary to provide a comprehensive idea of technical and economic feasibility of lignite pre-dying. The major objective of this study is thereby to be a demonstration of a detailed thermo-economic analysis of flue gas mill pre-drying concept. It is continuation of works associated with lignite pre-drying power systems.

2. Materials and Methods 2.1. System Layouts A conceptual schematic of the FPLPS system is depicted in Fig. 1. The common features of the FPLPS include the use of flue gas as drying agent in the drying tubes and fan mills, the integration of an open pulverizing system with the boiler, the concentration of water vapor in the dryer exhaust gas, and the air pre-heating by the dryer exhaust gas. More specifically, the flue gas dryer exhaust gas, containing a large amount of water vapor, is not recycled to the furnace but led to the heat recovery system, in which the waste heat is utilized to pre-heat the air supply of the boiler.

Fig. 1. System layout of the FPLPS.

2.2. Thermo-economic analysis model Raw lignite is fed into the dryer, in which a part of the moisture content is removed and the heat supply is basically conserved with the latent heat of moisture evaporation △M·hv. Accordingly, the LHV of the pre-dried lignite is derived as:

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pc ar vLHV (LHV ) (1 )M h M∆ ∆= + ⋅ − (1) where ΔM is the mass of evaporated moisture per kg raw lignite (kg·kg-1), also defined as pre-drying degree, and hv is the water latent heat of vaporization.

ar pc pc( ) (100 )M M M M∆ = − − (2) where Mar and Mpc are the moisture contents in the raw and pre-dried lignite, respectively. The moisture content of pre-dried lignite can be calculated as:

0.46pc ar 90 de0.048M M R t −= ⋅ ⋅ ⋅ (3)

In the present work, a dimensionless pre-drying upgrade factor is defiend as:

pd v ar1 LHVk M h= + ∆ ⋅ (4) In this way, boiler efficiencies on different basis can be derived as:

ref 0 ref apb, pd b,t t t tkη η= == ⋅ (5)

The plant thermal efficiency, ηtot, can be calculated by the combined product of the efficiencies of the main components, and is expressed as follows:

tot b p sc m gη η η η η η= (6) where ηb is the boiler efficiency (t0 basis), ηp the heat supply pipe efficiency, ηsc the steam cycle efficiency, ηm the mechanical efficiency, and ηg the electric generator efficiency. The plant thermal efficiency improvement, △ηtot, is calculated as:

tot tot FPLPS tot CLPS, ,∆η η η= − (7)

3. Results and Discussion 3.1. Case Study 3.1.1. Benchmark parameters To evaluate the energy saving potential of the FPLPS comprehensively, simulation calculations on a 600 MW water-cooled supercritical unit were carried out based on the simulation algorithm introduced above. Detailed specifications in simulation and calculation, such as the thermal parameters of the heat regenerative system, can be found in Ref.[21]. The properties of the Chinese Yimin lignite, with 39.5% moisture content are given in Table 1.

Table 1. Properties of Chinese Yimin lignite Mar, % Aar, % Car, % Har, % Oar, % Nar, % Sar, % LHV, MJ·kg-1 39.50 12.09 34.59 2.03 11.30 0.35 0.14 11.79

To conduct thermodynamic analysis, it is firstly necessary to determine the moisture content of pre-dried lignite with given parameters. According to the empirical correlation (Eq.1), the moisture content after grinding and milling can be obtained, as a function of pre-drying degree and R90 (Fig. 2). Mpc is calculated as 9.82% when tde equals 110 °C. The heating value can also be calculated, as shown in Fig. 3. The HHV and LHV are elevated by 49.1% and 59.5%, respectively. Obviously, with higher pre-drying degree, the rank of

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the lignite is improved. The plant efficiency benefit is expected. As summarized in Table 2, compared with conventional lignite-fired power system (CLPS), the plant thermal efficiency can be improved by 1.48 %-pts.

50 60 70 80 90 100 110 120 130 140 150

6

8

10

12

14

16

tde/°C

Mpc

/wt%

R90=50% R90=45% R90=40% R90=35%

Design point，Mpc=9.82%

ΔM/kg·kg-1

Hea

ting

Val

ue/M

J·kg-1

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.3511

13

15

17

19

21

HHVar

LHVar

HHVar,pc 1-ΔM=

=LHVar,rc+2510ΔM

1-ΔM

100-Mpc= Mar-MpcΔM

HHVar,rc

LHVar,pc

Raw Lignite1 kg

Pre-Dried Lignite(1-ΔM) kgDryer

Heat Source

Waste Heat

Moisture ContentMar kg

Moisture Content(1-ΔM)·Mpc kg

Moisture VaporΔM kg

Total Residual

Evaporated

HHVar kJLHVar kJ

HHVar kJLHVar+ΔM·hv kJ

Fig. 2. Mpc in terms of R90 and tde. Fig. 3. Heating value in terms of ΔM.

Table 2. Comparison of the parameters and thermo-economics of the CLPS and FPLPS Items Unit CLPS FPLPS

Boiler exhaust temperature, tbe °C 144 110 Air pre-heating temperature, tap °C - 55 Dryer exhaust temperature, tde °C 150 110

Boiler efficiency on tap basis, ref apb,t tη = % - 89.49

Pre-drying upgrade factor, kpd - - 1.07 Boiler efficiency on t0 basis,

ref 0b,t tη = % 92.59 95.76

Steam cycle efficiency, ηi % 47.67 47.67 Plant thermal efficiency, ηtot % 43.26 44.74

3.1.2. Thermodynamic analysis The joint influences of boiler exhaust temperature (tbe), dryer exhaust temperature (tde), air pre-heating temperature (tap) on the energy efficiency improvements are depicted in Fig. 4. It can be seen in Fig. 5 that the plant efficiency improvement varies with tbe, tde, and tap linearly. The △ηtot increases by 0.09 %-pts when tde decreases by 10 °C, 0.18 %-pts when tbe decreases by 10 °C, and 0.16 %-pts when tap increases by 10 °C. To sum up, the decrease of tde, tbe and increase in tap contribute to the total △ηtot by 0.38 %-pts, 0.62 %-pts, and 0.48 %-pts, respectively. The pre-drying upgrade factor (kpd) is defined in the present work to demonstrate the energy saving mechanism of pre-drying from energy analysis perspective. It is shown in Fig. 6 that the kpd increases linearly with the pre-drying degree (ΔM) with given lignite properties. It is calculated as 1.07 for Yimin lignite, which means that the boiler efficiency is increased by 7%. The kpd increases by 0.02 with 0.1 kg·kg-1 increase in ΔM. As a result, the boiler efficiency increases with the pre-drying degree (Fig. 7).

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Boiler

ECO

APH2 APH1

tbe

tde

tap t0

DRYER Ambient Air

Thg=const

Raw Lignite

Pre-dried Lignite

ap-air pre-heatingbe-boiler exhaust gasde-dryer exhaust gas

Mar=39.5% Mpc=9.82%

110115

120125

130

135

140

145

0.0

0.5

1.0

1.5

110

115

120

125130

135140

145150

Δηto

t/%-p

ts

t de/°Ctbe /°C

55 45 35 25

tap/°C

Fig. 4. Schematic of the key temperatures in FPLPS. Fig. 5. Influence of tbe, tde, and tap on Δηtot

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.351.00

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

Design point：ΔM=0.33 kg·kg-1

kpd=1.07

ΔM/kg·kg-1

k pd

Mar=39.5%

0.10 0.15 0.20 0.25 0.30 0.35

88.0

88.5

89.0

89.5

90.0

90.5

91.0

91.5

92.0

η b o

n t ap

bas

is/%

ΔM/kg·kg-1

ηb on tap basis

93.7

93.8

93.9

94.0

94.2

94.3

94.4

94.5

ηb on t0 basis

η b o

n t 0

basi

s/%

94.1

tbe=144 °C

Fig. 6. kpd as a function of ΔM. Fig. 7. Boiler efficiencies in terms of ΔM.

3.2. Parametric analysis Apart from the key temperatures above, the plant efficiency improvement also depends on drying process parameters. Pre-drying degree and dryer efficiency are the parameters, from the perspectives of moisture removal and thermal conversion, respectively. The uncertainty in the calculation of the moisture content should be taken into consideration, as well as the dryer efficiency. Therefore, it is necessary to investigate the variations of the improvement. 3.2.1. Influence of pre-drying degrees It is shown in Fig. 8 that the Δηtot increases ΔM linearly. Moreover, the slope increases with tbe. It means that for lignite-fired boilers with high exhaust temperature, the energy saving potential is remarkable. For example, when tbe is equal to 144 °C and 110 °C, respectively, the △ηtot increases by 0.12 %-pts and 0.05 %-pts, with 0.1 kg·kg-1 increase in ΔM. 3.2.2. Influence of dryer efficiency The dryer efficiency influences the energy saving potential significantly, as shown in Fig. 9. When the dryer efficiency drops below the design value, the Δηtot decreases sharply. Especially, there exit critical dryer efficiencies with given boiler exhaust temperature. For example, when tbe is equal to 144 °C, the ηd,cri is approximately 80%. When the dryer efficiency gets lower than that, the FPLPS does not save coal any more. It can be attributed to the fact that although kpd does not change with dryer efficiency, the heat load of the dryer increases with decreasing dryer efficiency and the boiler efficiency drops.

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0.10 0.15 0.20 0.25 0.30 0.350.5

0.7

0.9

1.1

1.3

1.5

tbe/°C

110 115 120 125 130 135 140 144

ΔM/kg·kg-1

Δηto

t/%-p

ts

70 75 80 85 90 95 100

0.0

0.3

0.6

0.9

1.2

1.5

1.8

110 115 120 125 130 135 140 144

ηd/%

tbe/°C

Δηto

t/%-p

ts

Fig. 8. Δηtot as a function of ΔM. Fig. 9. Δηtot as a function of ηd.

4. Conclusions A theoretical framework of modeling and thermo-economics on lignite pre-drying is presented in the present work, which offers a comprehensive idea of economic feasibility of lignite utilization integrated with flue gas fan mill pre-drying. The main conclusions are: 1) A pre-drying upgrade factor was defined to decouple the drying and boiler systems. When the moisture content decreased from 39.5% to 9.82%, the pre-drying degree was 0.33 kg·kg-1, and pre-drying upgrade factor was calculated to be 1.07. As a result, the plant thermal efficiency improvement of the FPLPS compared with CLPS can be up to 1.48 %-pts, when the boiler exhaust gas temperature, the dryer exhaust gas temperature and the air pre-heating temperature were set as 110 °C, 110 °C, and 55 °C, respectively. 2) Through parametric analyses, the contributions of key design temperatures and pre-drying conditions to the energy saving potential of the FPLPS were analyzed. Results showed that the plant efficiency improvement resulted from pre-drying and air supply pre-heating increased by 0.09 %-pts, 0.16 %-pts, and 0.18 %-pts with 10 °C increase in the dryer exhaust temperature, 10 °C decrease in the air pre-heating temperature, and 10 °C decrease in the boiler exhaust temperature, respectively. 5. Acknowledgments This work was supported by National Natural Science Foundation of China (NO. 51436006), the National Basic Research Program of China (973 Program, NO.2015CB251504), and the Fundamental Research Funds for the Central Universities. 6. References [1] Pusat, S.; Akkoyunlu, M.T.; Erdem, H.H. Evaporative drying of low-rank coal. In Sustainable

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thermo-economic analysis of an efficient lignite-fired

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